Method for operating a dosing apparatus and motor vehicle having a dosing apparatus

ABSTRACT

A method for operating a dosing apparatus for providing a liquid additive includes providing the dosing apparatus with at least one pump for pumping the additive from a tank into a pressure accumulator, a dosing valve for providing additive present in the pressure accumulator in a dosed manner, and a return valve through which additive present in the pressure accumulator can be led back into the tank. A dosing demand is established, subsequently the pump is activated to build up a pressure in the pressure accumulator, then the pressure in the pressure accumulator is set to a desired metering pressure, and thereafter the liquid additive is dispensed by the dosing valve. A motor vehicle having a dosing apparatus is also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation application, under 35 U.S.C. §120, of copendinginternational application PCT/EP2013/052044, filed Feb. 1, 2013, whichdesignated the United States; this application also claims the priority,under 35 U.S.C. §119, of German patent application DE 10 2012 002 059.7,filed Feb. 3, 2012; the prior applications are herewith incorporated byreference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for operating a dosing or meteringapparatus for providing a liquid additive. The dosing apparatus may, forexample, be utilized for feeding a liquid additive into an exhaust-gastreatment apparatus, where the liquid additive is used for thepurification of the exhaust gases of a mobile internal combustion engine(in particular in a motor vehicle). The invention also relates to amotor vehicle having a dosing or metering apparatus.

Exhaust-gas treatment apparatuses into which a liquid additive is fedhave become increasingly widely used in recent times. An exhaust-gaspurification method particularly commonly implemented in suchexhaust-gas treatment apparatuses is the selective catalytic reduction(SCR) method in which nitrogen oxide compounds in the exhaust gas arereduced with the aid of a reducing agent. The reducing agent isgenerally fed to the exhaust-gas treatment apparatus in the form of aliquid additive. A liquid additive particularly commonly used in thiscontext is urea-water solution. A urea-water solution with a ureacontent of 32.5% is available under the trademaek AdBlue® and is widelyused. The urea-water solution is merely a reducing agent precursor andis converted into ammonia (the reducing agent) outside the exhaust gas(in a reactor provided for that purpose) or in the exhaust gas (in theexhaust-gas treatment apparatus, under the action of the exhaust gas).Nitrogen oxide compounds in the exhaust gas are then, together with theammonia, reduced in the presence of an SCR catalytic converter to formnon-hazardous substances (water, CO₂ and nitrogen). The invention isused in this technical field in particular.

The dosing apparatus for providing a liquid additive (urea-watersolution) should be of the simplest possible construction and should beas durable, maintenance-free and inexpensive as possible. At the sametime, it is desirable for the dosing accuracy of a dosing apparatus tobe particularly high. In this way, it is possible firstly for the amountof liquid additive required for the conversion of pollutant constituentsin the exhaust gas to be set in a particularly exact manner. It isfurthermore possible for overdosing to be prevented in an effectivemanner.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a method foroperating a dosing apparatus and a motor vehicle having a dosingapparatus, which overcome the hereinafore-mentioned disadvantages andsolve or at least alleviate the highlighted technical problems of theheretofore-known methods and vehicles of this general type. It issought, in particular, to propose an especially advantageous method foroperating a dosing apparatus for urea-water solution.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a method for operating a dosing apparatusfor providing a liquid additive, in particular urea-water solution, themethod comprising at least the following steps:

providing the dosing apparatus with at least the following:

-   -   at least one pump for delivering the additive from a tank into a        pressure accumulator,    -   a dosing valve which is constructed for providing additive        present in the pressure accumulator in a dosed manner, and    -   a return valve through which additive present in the pressure        accumulator can be discharged back into the tank;

-   a) identifying a dosing demand,

-   b) activating the pump in order to build up a pressure in the    pressure accumulator,

-   c) setting the pressure in the pressure accumulator to a dosing    pressure, and

-   d) dispensing the additive through the dosing valve.

The dosing apparatus particularly preferably serves for the dosing of areducing agent (or of a reducing agent precursor such as urea-watersolution) as a liquid additive into an exhaust-gas treatment apparatusof an internal combustion engine. The pump is preferably a diaphragmpump or a piston pump. The delivery rate of the pump is preferably notregulated. This means that an electronic regulator or controller,through the use of which the flow rate of additive delivered by the pumpcan be exactly set, is not provided. The tank is preferably connected tothe pump through a suction line through which the pump can draw liquidadditive out of the tank. The pressure accumulator is preferablydisposed downstream of the pump as viewed in a flow direction of theliquid additive from the tank to the dosing valve. A pressure built upby the pump then prevails in the pressure accumulator. The pressureaccumulator may, for example, be in the form of a flexible line thatexpands when liquid additive is delivered into the flexible line underpressure by the pump. The dosing valve is preferably an electricallycontrolled solenoid valve that can be opened and closed by the electricdrive, wherein the opening time of the dosing valve predefines theamount of liquid additive provided. The pressure accumulator is alsoadjoined by a return valve. The return valve is preferably connectedback to the tank through a return line in order to ensure that liquidadditive present in the pressure accumulator can be discharged back intothe tank.

The identification of a dosing demand in step a) preferably involves theidentification of an electronic signal from an engine controller, whichelectronic signal is representative of a dosing demand. For example, anengine controller transmits a signal that corresponds to a certain flowrate of liquid additive required. The signal is detected, or identified,as a dosing demand. During step a), it is preferably the case that apressure prevailing in the pressure accumulator is so low that handlingof the dosing demand is not possible. It is preferably the case that,during step a), the pressure in the pressure accumulator is lower than 2bar, particularly preferably lower than 1 bar and very particularlypreferably lower than 0.5 bar. In step b), the pump is activated inorder to build up a pressure in the pressure accumulator that isrequired in order to ensure that suitable dosing can be performed byusing the dosing valve. In order to build up the pressure, a diaphragmpump or a piston pump is preferably operated with between 2 and 10 pumpstrokes. The number of pump strokes required to build up the requiredpressure in the pressure accumulator is dependent on the flexibility ofthe pressure accumulator and on the pressure difference between thepressure during step a) and the dosing pressure (step b). The moreflexible the pressure accumulator, the more liquid additive can bedelivered into the pressure accumulator in order for the requiredpressure to be built up. The greater the pressure difference, the moreliquid additive can be delivered into the pressure accumulator. Thepressure built up as a result of the activation of the pump in step b)is preferably between 3 and 10 bar, particular preferably between 5 and10 bar and very particularly preferably between 6 and 8 bar. Thepressure built up by the pump in step b) is typically slightly higherthan the dosing pressure required in order to be able to perform precisedosing by using the dosing valve. The reason for this is that the pumpis preferably unregulated. This means that the pump is not deactivatedwhen a certain pressure is attained, and instead, the pump, after beingactivated, initially continues running regardless of the magnitude ofthe pressure prevailing in the pressure accumulator, which pressure actscounter to the pump.

In one structural variant of the described method, the operation of thepump ends only when the pressure in the pressure accumulator is so highthat the pump can generate no further pressure increase in the pressureaccumulator.

In accordance with another particularly preferable mode of the method ofthe invention, method steps a) to d) are performed repeatedly during theoperation of the dosing apparatus.

In accordance with a further preferable mode of the method of theinvention, in order to set the pressure in step c), the pressure isreduced, in particular by virtue of the return valve being opened. It isthus preferably provided that, in order to set a precise pressure in thepressure accumulator that is desired for being able to perform veryprecise dosing by using the dosing valve, the return valve is opened.

In accordance with an added preferable mode of the method of theinvention, step b) continues to take place during step c). The returnvalve remains open and the pump is activated. Due to a return flow ofliquid additive through the return valve, the pressure in the pressureaccumulator remains constant.

In accordance with an additional preferable mode of the method of theinvention, at least steps c) and d) are performed at least partially inparallel with one another. It is particularly preferable even for stepsb), c) and d) to take place at least partially in parallel (at the sametime).

The dosing valve is opened for the dispensing of the liquid additivethrough the dosing valve in step d). Liquid additive then flows out ofthe pressure accumulator through the dosing valve to a consumer for theliquid additive. The consumer for the liquid additive is preferably anexhaust-gas treatment apparatus in which the method of selectivecatalytic reduction is performed using the liquid additive. It ispreferably the case that method steps b) and c) continue to take placeduring the execution of step d). During step d), the pump remainsactivated and continues to deliver liquid additive into the pressureaccumulator. It is preferably also the case that, during step d), thereturn valve remains open and ensures that the pressure in the pressureaccumulator is set to the dosing pressure, required for the dosingaction, of for example a value between 5 and 10 bar, for example 7 bar.In this case, the return valve is preferably not open at all times, andis instead opened in pressure-controlled fashion every time the pressurein the pressure accumulator exceeds the dosing pressure. In this way,the pressure in the pressure accumulator is in each case repeatedlyregulated to the dosing pressure during the dispensing of the additivethrough the dosing valve.

It is not necessary for the dosing valve to be open continuously fordispensing the additive in step d). It is also possible for the dosingdemand identified in step a) to be dispensed in several pulses. In thiscase, a pulse corresponds in each case to one opening process and oneclosing process of the dosing valve.

If the method is executed repeatedly during the operation of the dosingapparatus, it is preferably the case that the pump is deactivated aftermethod step d) in each case. The pump is then reactivated only when anew dosing demand is identified in a new step a). In particular, thepump does not run continuously during the operation of a delivery lineand of a motor vehicle in which the dosing apparatus is disposed, andinstead, the pump is generally activated only when a dosing demand ispresent.

In accordance with yet another advantageous mode of the method of theinvention, a step e) of lowering the pressure in the pressureaccumulator is performed after method step d). Method step e) ispreferably also performed repeatedly together with method steps a) tod).

As a result of the lowering of the pressure in the pressure accumulatorin step e), it is made possible for the pressure accumulator to berelieved of load between two dosing demands. The durability of thedosing apparatus can be increased in this way. In one structuralvariant, the pressure accumulator may be actively relieved of load. Areturn valve that branches off from the pressure accumulator may, ifappropriate, be actively opened in step e) in order to permit an escapeof the liquid additive from the pressure accumulator and thus ensure alowering of the pressure in the pressure accumulator. The return valvemay be identical to the return valve used in step c) of the method, andmay be additionally capable of being actively opened in step e). It may,however, also be provided that, for step e), an additional return valveis provided which can be actively opened and which is disposedpreferably in a parallel second return line from the pressureaccumulator to the tank.

In a further structural variant of the method, it is also possible foractive relief of load to be effected by using the pump of the deliveryunit. In step e), the pump may, if appropriate, be operated counter tothe delivery direction in order to deliver liquid additive out of thepressure accumulator and back into the tank in order that the pressurein the pressure accumulator decreases and the pressure accumulator isthus relieved of load.

It is also possible for the pressure accumulator to be passivelyrelieved of load. For this purpose, it is possible for a leakage flow ofliquid additive to flow back into the tank through a return valve and/orthrough the pump in step e), this resulting in a lowering of thepressure in the pressure accumulator. In the case of a passive relief ofload in step e), the pressure in the pressure accumulator fallsrelatively slowly because the leakage flow is generally relativelysmall. The effect of the leakage flow on the pressure in the pressureaccumulator during the dosing in step d) would otherwise be too great.

In accordance with yet a further advantageous mode of the method of theinvention, the return valve is a passively opening valve, the openingpressure of which corresponds to the dosing pressure.

The return valve is preferably a passively opening valve which opens ata predetermined or preset threshold pressure, wherein the thresholdpressure corresponds to the dosing pressure (the pressure with which theadditive is dosed). The return valve preferably has a valve body and avalve spring which exerts a preload on the valve body. The return valveopens when the force exerted on the return valve by the liquid additivepresent in the pressure accumulator exceeds the spring force of thespring in the return valve. The use of a return valve of that type makesit possible to realize a particularly inexpensive dosing apparatusbecause no (electric) drive is required for the dosing valve, and also,no control unit is required for the control of the return valve.

The dosing apparatus can be implemented without an electrically drivendosing valve and/or without an (active) control unit of the returnvalve.

In accordance with yet an added advantageous mode of the method of theinvention, the return valve is an actively opening valve with a valvedrive, wherein a pressure sensor is disposed on the pressure accumulatorand the return valve is controlled, with the aid of the valve drive, bya control unit in order to set the pressure in the pressure accumulatorto the desired dosing pressure in step c).

The valve drive may, for example, be an electromagnet which exerts aforce on an armature in the return valve, through the use of which forcethe return valve can be opened and/or closed. The pressure sensor may,for example, be in the form of an electronic pressure sensor whichmeasures a pressure in the pressure accumulator and transmits it as anelectrical signal to the control unit. The pressure information is thenprocessed in the control unit in order to specify whether the check orreturn valve is to be opened and/or closed (regulating loop).

In accordance with yet an additional advantageous mode of the method ofthe invention, the pump has a pump chamber and at least one pump valvewhich predefines a delivery direction. The pump chamber preferably has apump movement imparted to it by a diaphragm or a pump piston. In apreferred embodiment, two pump valves are provided each of which has apassively opening configuration and is respectively disposed upstream ordownstream of the pump chamber as viewed in the flow direction of theliquid additive from the tank to the dosing valve.

In a further embodiment, only a single pump valve is provided. Thesingle pump valve is then preferably provided downstream of the pumpchamber as viewed in the flow direction. The pump piston is thenconstructed in such a way that, during a delivery movement (dischargemovement), it discharges the liquid additive in the pump chamber throughthe pump valve. A replenishment flow of liquid additive into the pumpchamber then takes place during a return movement of the pump pistondirected counter to the delivery movement. A pump of that type isdescribed, for example, in German Patent DE 10 2008 010 073 B4, inparticular in FIGS. 2 and 3 and in the explanations with regard theretoin paragraphs [0038] to [0051], the entire content of which isincorporated herein.

The method according to the invention is used, in particular, in thecase of a dosing apparatus that preferably does not have a pressuresensor on the pressure accumulator for electronically monitoring thepressure in the pressure accumulator. In this case, the pressure in thepressure accumulator is set exclusively by using the return valve. Thereturn valve preferably acts mechanically.

With the objects of the invention in view, there is concomitantlyprovided a motor vehicle, comprising an internal combustion engine, anexhaust-gas treatment apparatus for purifying exhaust gases of theinternal combustion engine and a dosing apparatus through which a liquidadditive can be fed to the exhaust-gas treatment apparatus and which isset up and constructed for operation in accordance with the describedmethod. The motor vehicle is preferably a passenger motor vehicle or atruck. The internal combustion engine is preferably a diesel internalcombustion engine. The exhaust-gas treatment apparatus preferably has anSCR catalytic converter for carrying out the method of selectivecatalytic reduction. The exhaust-gas treatment apparatus is preferablyfed with reducing agent, and in particular urea-water solution as aliquid additive, upstream of the SCR catalytic converter. Then, themethod of selective catalytic reduction is performed with the reducingagent in the exhaust-gas treatment apparatus for the purpose of reducingnitrogen oxide compounds in the exhaust gas in an effective manner.

Other features which are considered as characteristic for the inventionare set forth in the appended claims, noting that the features specifiedindividually in the claims may be combined with one another in anydesired technologically meaningful way and may be supplemented byexplanatory facts from the description, with further structural variantsof the invention being specified.

Although the invention is illustrated and described herein as embodiedin a method for operating a dosing apparatus and a motor vehicle havinga dosing apparatus, it is nevertheless not intended to be limited to thedetails shown, since various modifications and structural changes may bemade therein without departing from the spirit of the invention andwithin the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a block diagram of a dosing apparatus together with a tank andan exhaust-gas treatment apparatus;

FIG. 2 is a block diagram of a further structural variant of a dosingapparatus together with a tank and an exhaust-gas treatment apparatus;

FIG. 3 is a block diagram of a motor vehicle having a dosing apparatus;

FIG. 4 is a first diagram showing a sequence of the method according tothe invention; and

FIG. 5 is a second diagram showing the sequence of the method accordingto the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures of the drawings in detail and first,particularly, to FIG. 1 thereof, there is seen a dosing apparatus 1together with a tank 3 and an exhaust-gas treatment apparatus 11. Thedosing apparatus 1 extracts liquid additive (urea-water solution) fromthe tank 3 at an extraction point 17. A delivery line 18 which extendsfrom the extraction point 17 runs initially to a pump 2. The pump 2 hasa pump chamber 22 and at least one pump valve 23 predefining a deliverydirection 24. The pump chamber 22 has a diaphragm or a pump piston andmay have only one valve downstream of the pump chamber, as describedfurther above. The delivery line 18 extends from the pump 2 onward to adosing valve 5, through which the liquid additive can be fed into anexhaust-gas flow 13 in the exhaust-gas treatment apparatus 11. Theliquid additive is delivered from the tank 3 through the delivery line18 to the dosing valve 5 due to the delivery action of the pump 2. Apressure accumulator 4 is situated downstream of the pump 2, as viewedin the delivery direction from the tank 3 to the dosing valve 5. Thepressure accumulator 4 may be formed in part by the delivery line 18,for example by virtue of the delivery line 18 being in the form of aflexible hose. A return line 12 branches off from the pressureaccumulator 4 and leads back into the tank 3. A return valve 6 isdisposed in the return line 12. Through the use of a dosing apparatus ofthis type, the method according to the invention can be carried out in aparticularly advantageous manner.

The structural variant of a dosing apparatus according to FIG. 2corresponds, for the most part, to the structural variant according toFIG. 1. A valve drive 19 with which the return valve 6 can be opened andclosed, is additionally provided in this case. The pressure accumulator4 is adjoined by a pressure sensor 20 with which the pressure in thepressure accumulator 4 can be measured. The pressure informationdetermined by using the pressure sensor 20 is passed to a control unitor controller 21 and processed therein. The control unit 21 can open andclose the return valve 6 as required by using the valve drive 19.

FIG. 3 shows a motor vehicle 9 having an internal combustion engine 10and an exhaust-gas treatment apparatus 11 with which the exhaust gasesof the internal combustion engine 10 can be purified. The exhaust gasesof the internal combustion engine 10 flow, as an exhaust-gas flow 13,through the exhaust-gas treatment apparatus 11. A liquid additive can befed to the exhaust-gas treatment apparatus 11 through a dosing valve 5provided on the exhaust-gas treatment apparatus 11. The liquid additiveis supplied from a tank 3 to the dosing valve 5 by a dosing apparatus 1.

FIG. 4 shows a first flow diagram of the method according to theinvention. A time axis 14 and a pressure axis 15 of the diagram can beseen. A pressure profile 16 which occurs during the method according tothe invention is plotted against the time axis 14. The pressure profile16 is representative of the pressure in the pressure accumulator of thedescribed dosing apparatus. In step a), the pressure in the pressureaccumulator is at a low constant rest pressure level, which is forexample less than 2 bar. During step a), a dosing demand is identified.If a dosing demand has been identified, then in step b), the pressure inthe pressure accumulator is increased, as described further above, byactivation of the pump. The pressure profile 16 rises sharply. In stepb), the pressure is, in part, increased to such an extent that itexceeds a dosing pressure 8. The pressure is set to the dosing pressure8 by opening the return valve in step c), as described further above.Then, in step d), dispensing of the liquid additive is performed throughthe dosing valve, with the liquid additive being at the dosing pressure8. Subsequently, step e) is performed, in which the pressure in thepressure accumulator falls again. The pressure profile 16 is preferablyreduced to a rest pressure 7 from step a) again. An active execution ofstep e) (for example by an active opening of a return valve, or a returndelivery action by the pump) is not required. It is also adequate if, instep e), the pressure falls passively (for example due to a leakage flowthrough a return valve or through the pump).

FIG. 5 illustrates the method steps a), b), c), d) and e). It can beseen that the method steps are performed repeatedly at regular intervalsin the manner of a loop.

1. A method for operating a dosing apparatus for providing a liquidadditive, the method comprising the following steps: providing thedosing apparatus with at least one pump configured to deliver theadditive from a tank into a pressure accumulator, a dosing valveconfigured to provide additive present in the pressure accumulator in adosed manner, and a return valve configured to discharge the additivepresent in the pressure accumulator back into the tank; a) identifying adosing demand; b) activating the at least one pump to build up apressure in the pressure accumulator; c) setting the pressure in thepressure accumulator to a dosing pressure; and d) dispensing theadditive through the dosing valve.
 2. The method according to claim 1,which further comprises performing steps a) to d) repeatedly in a loopduring operation of the dosing apparatus.
 3. The method according toclaim 1, which further comprises opening the return valve for settingthe pressure in step c).
 4. The method according to claim 1, whichfurther comprises performing at least steps c) and d) at least partiallyin parallel with one another.
 5. The method according to claim 1, whichfurther comprises performing the following step after step d): e)lowering the pressure in the pressure accumulator.
 6. The methodaccording to claim 1, which further comprises providing the return valveas a passively opening valve having an opening pressure corresponding tothe dosing pressure.
 7. The method according to claim 1, which furthercomprises: providing the return valve as an actively opening valvehaving a valve drive; providing a pressure sensor on the pressureaccumulator; and controlling the return valve using the valve drive anda controller to set the pressure in the pressure accumulator to thedosing pressure in step c).
 8. The method according to claim 1, whichfurther comprises providing the pump with a pump chamber and at leastone pump valve predefining a delivery direction.
 9. A motor vehicle,comprising: an internal combustion engine; an exhaust-gas treatmentapparatus configured to purify exhaust gases from said internalcombustion engine; a tank configured to store a liquid additive; and adosing apparatus configured to feed the additive from said tank to saidexhaust-gas treatment apparatus; said dosing apparatus including apressure accumulator, at least one pump configured to deliver theadditive from said tank into said pressure accumulator, a dosing valveconfigured to provide additive present in said pressure accumulator in adosed manner, and a return valve configured to discharge the additivepresent in said pressure accumulator back into the tank; and said dosingapparatus configured to: a) identify a dosing demand; b) activate saidat least one pump to build up a pressure in said pressure accumulator;c) set the pressure in said pressure accumulator to a dosing pressure;and d) dispense the additive through said dosing valve.